Contrasting arc magma fertilities in the Gangdese belt, Southern Tibet: Evidence from geochemical variations of Jurassic volcanic rocks

Abstract

Jurassic volcanic rocks are widely distributed along the Gangdese belt of southern Tibet, forming a westerly-extending southern belt (Bima Formation) with one large arc-related porphyry CuAu deposit (Xietongmen), and an easterly-extending northern belt (Yeba Formation) without any contemporary mineralization. A compilation of new and published data on the Bima Formation is compared to published data on the Yeba Formation in order to understand their different relationships with mineralization. The Bima Formation volcanic rocks comprise basalt, andesite, and dacite with zircon UPb ages of 195.0 Ma–165.1 Ma (including new ages of 180.2 ± 0.8 Ma to 166.7 ± 1.1 Ma). They are calc-alkaline series lava flows and show enrichments in LILE and LREE, depletions in HFSE and HREE, which are features typical of arc rocks. Coherent geochemical variations and similar depleted Sr-Nd-Hf isotopic characteristics between basaltic, intermediate, and felsic rocks (new and published data: (87Sr/86Sr)i = 0.703182 to 0.705489); εNd(t) = 3.2 to 7.1; εHf(t) = 9.3 to 16.0) suggest that they were formed from fractional crystallization of the basaltic magma. In contrast, the published Yeba Formation volcanic rocks, although with a similar age range (192.7 Ma to 168.0 Ma), have a wider compositional range (from basalt to rhyolite), and more evolved Sr-Nd-Hf isotopic compositions ((87Sr/86Sr)i = 0.700182 to 0.707723; εNd(t) = −5.1 to 4.5; εHf(t) = −18.5 to 17.6).New data on the Bima Formation andesite and dacite samples have higher zircon Eu/Eu*, Ce4+/Ce3+ and (Ce/Nd)/Y ratios, and lower titanium-in-zircon temperatures than those from the Yeba Formation, suggesting the Bima Formation magmas were more oxidized and hydrous, and therefore more fertile for porphyry copper deposit formation. The relatively reduced Yeba Formation magmas formed in a distal arc setting near the Central Lhasa old basement. Their evolved isotopic characteristics and reduced nature may have resulted from more extensive contamination by reduced crustal components, rendering them infertile for porphyry copper mineralization. Exploration for arc-related porphyry copper deposits in the Gangdese belt should therefore focus on the fertile Bima Formation.